How Can I Find Out Which Site Millions Of Usernames And Passwords Are For?

[gokansen]

I have the username:pass combo list but idk how do I use them? How can I find out which site millions of usernames and passwords are for?

 

[fringill]

Try login to your favorite sites, see if they work .
Or more realistically, you could maybe use the lists to build up hash-cracking dictionaries, or assume that the username:pass combo will be used on various sites and try your luck?